Power converters are used to convert electric power from one form to another, for example, to convert direct current (DC) power to alternating current (AC) power. One important application for power converters is in transferring power from energy sources such as solar panels, batteries, fuel cells, etc., to electric power distribution systems such as local and regional power grids. Most power grids operate on AC current at a line (or mains) frequency of 50 or 60 cycles per second (Hertz or Hz). Power in an AC grid flows in a pulsating manner with power peaks occurring at twice the line frequency, i.e., 100 Hz or 120 Hz. In contrast, many energy sources supply DC power in a steady manner. Therefore, a power conversion system for transferring power from a DC source to an AC grid typically includes some form of energy storage to balance the steady input power with the pulsating output power.
This can be better understood with reference to FIG. 1 which illustrates the mismatch between a DC power source and a 60 Hz AC load. The maximum amount of power available from the DC source is shown as a constant value. In contrast, the amount of power that must be transferred to the AC load fluctuates from zero to a maximum value and back down to minimum once every 8.33 milliseconds (ms). During time T1, the power available from the DC source exceeds the instantaneous power required by the AC load. During time T2, however, the maximum power available from the DC source is less than that required by the load. Therefore, to effectively transfer power from the source to the load, the power conversion system must store the excess energy from the power source during time T1 (shown as the shaded area S), and discharge the stored energy to the load during time T2 (shown as the shaded area D).
Energy storage devices for power converters tend to be expensive, bulky, unreliable, and inefficient. These factors have been barriers to large-scale adoption of alternative energy sources such as solar and fuel cells which generate electricity in the form of DC power. They have also been barriers to large scale adoption of back-up power systems for computers, residences, schools, businesses, etc.
The cost and reliability factors have been especially critical for solar energy systems. Solar panel makers have improved the reliability of their products to the point that 20-year warranties are common. Manufacturers of power converters, however, have not reached a point where they can offer warranties that are comparable to those for solar panels.